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This Weekend Sky At A Glance, December 20 – 29

 

Sirius and Procyon in the balance. Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime. Procyon, the Little Dog Star, shines to Sirius's left by about two fists at arm's length.

But directly left? Well, that depends. If you live around latitude 30° (Tijuana, Austin, New Orleans, Jacksonville), the two dog stars will be at the same height above your horizon soon after they rise. If you're north of that latitude, Procyon will be higher. If you're south of there, Sirius will be the higher one.

Why? Your eastern horizon tilts differently with respect to the stars depending on your latitude.

Saturn continues to close in toward Venus during and after twilight.
They form a big triangle with 1st-magnitude Fomalhaut. The triangle will be equilateral on Christmas night.

This is the shortest day of the year in the Northern Hemisphere; the longest day in the Southern Hemisphere. The solstice occurs at 4:20 a.m. EST, when the Sun reaches its farthest south declination and begins its six-month return northward. At sunrise I will be with a crowd of fellow oddballs from church singing up the Sun by a lakeside, just so it doesn't forget to begin its return. You can thank us for the coming spring and summer. Works every year.

How well do you really know the Orion's Belt region in binoculars? Can you piece out Orion's S, which begins and ends with the belt's two westernmost stars?

The Belt marks the rough center portion of the Orion OB1 Association, called OB1b. Are you aware of Orion OB1a and OB1c on either side of it? Binoculars are all you need, along with Matt Wedel's Binocular Highlight column and map in the January Sky & Telescope, page 43.

The little Pleiades cluster shines very high in the southeast after dinnertime, no bigger than your fingertip at arm's length. How many Pleiads can you count with your unaided eye? Take your time and keep looking. Most people can count 6. With extra-sharp eyesight, a good dark sky, and a steady gaze, you may be able to make out 8 or 9.

Right around the end of twilight, face north and look very high. Cassiopeia is now a flattened M canted at an angle, with its left side highest (depending on where you live). Just two hours later, the M is horizontal! Constellations passing near your zenith appear to rotate rapidly with respect to your direction "up."

Tonight at 8:48 p.m. EST, be watching Jupiter in your telescope. Jupiter's moon Io will slowly reappear out of eclipse from Jupiter's shadow, just to the planet's east.Europa looks on from farther east, while Ganymede and Callisto hang out on Jupiter's other side.

To the right of bright Jupiter shines orange Aldebaran, which comes with the large, loose Hyades cluster in its background. Binoculars are the ideal instrument for this cluster given its size: its brightest stars (magnitude 3.5 to 5) span an area about 4° wide. Higher above, the Pleiades are hardly more than 1° across counting just the brightest stars.

The main Hyades stars form a V, lying on its side these evenings. Aldebaran forms the lower of the V's two tips.

With binoculars, follow the lower branch of the V to the right from Aldebaran. The first thing you come to is the House asterism: a pattern of stars like a child's drawing of a house with a peaked roof. The house is currently upright and bent to the right like it got pushed.

The House includes three easy binocular double stars that form an equilateral triangle, with each pair facing the others. The brightest pair is Theta1 and Theta2 Tauri (the only members of the House that appear on the chart below). You may find that you can resolve the Theta pair with your unaided eyes.

Jupiter is nearly a month past its opposition. That means it's already fairly high in the east when you first catch sight of it through the fading twilight. How much later in twilight will you first see Aldebaran 6° to its right? And then the 3rd and 4th-magnitude stars of the Hyades V?

Right after dark, spot bright Venus in the southwest. Look just 1.1° to its left of for the 3rd-magnitude star Delta Capricorni. Binoculars will help. Although 3rd magnitude sounds easy naked-eye, Delta Cap is less than a thousandth as bright as Venus and less than a finger-width at arm's length from its glare!

As dawn brightens tomorrow morning, look very low in the southeast for the thin waning crescent Moon. Antares is only about 1° to the Moon's left, and brighter Mercury is 8° to its left, as shown below. Binoculars help.Antares is magnitude +1.0, while Mercury is magnitude –0.3, three times brighter.

 If you're in the Eastern time zone, early this evening Jupiter's satellites will look seriously out of kilter. We're used to seeing them line up on either side of Jupiter in a more or less straight line with it, like beads on a wire. But at 6:34 p.m. EST, you can catch Callisto, Europa, and Io forming a very straight line that's canted way out of whack, aiming away from Jupiter entirely!

Such things can happen because the plane of Jupiter's orbit is currently tipped slightly to our line of sight, allowing unusual perspectives to be presented to us. See Bob King's

As the year nears its end, Orion has fully come into its own. He's striding up the east-southeastern sky as soon as it gets dark, with his three-star Belt is nearly vertical. Left of the Belt is orange Betelgeuse and right of the Belt is bright white Rigel, supergiants both.

The Belt points up toward Aldebaran and Jupiter and, even higher, the Pleiades. In the other direction, it points down to where Sirius rises shortly after twilight's end.

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